A redundant cable changes the Layer 2 topology
Two fictional access switches and one distribution switch form an intended redundant triangle. The supplied classroom configuration uses a single spanning-tree instance for the VLAN being investigated. An operator enables the third cable after hours and immediately sees repeated broadcast traffic and a single source address moving between two ports. The team has no approved evidence that the redundant port is participating correctly in loop prevention. One colleague proposes disabling spanning tree everywhere to stop its messages; another wants to inspect the root election, port roles, allowed VLANs and relevant counters before choosing a controlled response. The task is a written diagnosis using the observations, not permission to issue commands on a live switch. Other causes of address movement remain possible, so the symptoms alone do not identify one cable conclusively.
1. Explain the plausible mechanism and the evidence needed.
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An unintended Layer 2 loop can replicate broadcasts and cause unstable source learning. Check the actual VLAN path, STP root and port roles, BPDU handling, link behavior and counter changes; address movement alone is not a complete diagnosis.
2. Why is globally disabling STP a poor generic remedy?
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Removing loop prevention can preserve or worsen a redundant Layer 2 loop. Any isolation or configuration change needs the actual topology, authorization, impact and recovery plan rather than a universal command based only on the symptom.